The present invention relates to a control circuit for an electronic switch, and more particularly to the control circuit for the high side switches of a bridge such as an H bridge, a half bridge, a three-chase bridge, or a full bridge. Such a bridge is typically used in a situation where the high side and low side switches alternatively conduct in counterposition to one another. Such bridges are especially useful to transfer direct current power from a bus to alternating current power in a load element or vice versa, as well as for performing full or half wave rectification. These bridges are also used extensively in audio amplifiers, motor controllers, DC/DC converters, noise cancellation systems, electronic lamp ballasts, and Uninterruptible Power Supply systems (UPS) applications.
As indicated, bridge circuits are well known and, as schematically illustrated for a H-bridge circuit in FIG. 1A, a half bridge circuit in FIG. 1B and a 3-phase bridge in FIG. 1C, include a circuit in which direct current from a source V may be selectively passed through a load through the operation of various high side and low side switches.
With reference to FIG. 1A by way of example, switches SW 1 and SW 4 in one direction and alternatively, through switches SW 3 and SW 2 in the opposite direction, thus alternating the current through the load. In such circuits, the operation of the switches SW 1-SW 4 are typically controlled by a relatively low voltage control circuit 10 illustrated in FIG. 1A for switch SW 1.
The control circuit 10 is typically provided with one or more sources of power from a power supply 12 and receives operating instructions from a logic circuit 14. In general, the control circuits for the two high side switches SW 1 and SW 3 are identical, as are the control circuits for the low side switches SW 2 and SW 4. However, the control circuits for the high side switches are not necessarily identical to the control circuits for the low side switches. The power supplies for the control circuits may be shared and a common logic circuit is conventional for ensuring that the switches SW 1-SW 4 operate simultaneously and in pairs.
One of the main design issues of the bridge topology is control of the high side switch since it is not ground referenced. With reference to FIG. 1A, the high side switch is referenced to VS which floats between values dependant upon which switches are conducting.
The design issues for the high side switch control circuit include power dissipation, noise immunity, propagation delay, and undervoltage protection. Power dissipation is important because even microamperes of current at the high side voltages can result is high power dissipation. Because of the desirability of controlling power dissipation in the control circuits, the output signals from the control circuits typically "latch" pending a further instruction from the associated logic circuit, and are hereinafter sometimes referenced as "pulse" control signals. Known latching circuits may be implemented with discrete components or on an integrated circuit ("IC").
While power is thus conserved, circuits which latch are susceptible to latching in an undesirable state under the influence of an external event such as transient noise or lightning, generally resulting in the destruction of the bridge circuit.
The lack of noise immunity is a concern because the large voltage transients associated with switching, ESD events, and even supply voltage transients can disrupt control of the bridge. In control circuits using discrete components, it is known to accept the power dissipation of continuous non-latching control signals (hereinafter sometimes referenced as "continuous" control signals) to control the operation of the high side switches. This technique is not used in ICs because it requires large IC devices, high power dissipation, and it results in slow switching speeds.
Propagation delay is also important because of the desirability of high frequency operation, and, more importantly, to be able to match the propagation delay of the high and low sides of the bridge to achieve balance across the load. The present invention, by initiating switching with short duration pulse signals, permits precise control of the timing without consuming excessive power.
Undervoltage protection circuitry is also required for the control circuits to insure switch operation, for high side switch conduction when there is inadequate power to control the low side switches can destruct the bridge. In the known prior art, the failure of the low side power supply results in the sending of a turn off or inhibit signal from the low side to the high side when a low side undervoltage condition existed. However, such solutions presumed that there was sufficient voltage to send the turn off signal, and were not fail safe.
It is accordingly an object of the present invention to obviate many of the foregoing problems and to provide a novel switch control circuit and method.
It is another object of the present invention to provide a novel method and integrated circuit for the non-latching, continuous control of the conduction of a high side switch.
Another object of the present invention is to provide a novel bridge with IC control circuitry for high side switch conduction control using continuous control signals.
Still another object of the present invention is to provide a novel method and circuit for controlling the high side switches of a bridge by a combination of pulse and continuous control signals, all without effecting a latch.
It is yet another object of the present invention to provide a novel method and fail safe undervoltage protection circuit.
A further object of the present invention is to provide a novel bridge with undervoltage protection for the circuits which control switch conduction.
These and many ocher objects and advantages of the present invention will be apparent to one skilled in the art from the claims and from the following detailed description when read in conjunction with the appended drawings.